Pirani Pressure Research Articles

Multi-layer graphene pirani pressure sensors

The operating principle of Pirani pressure sensors is based on the pressure dependence of a suspended strip’s electrical conductivity, caused by the thermal conductance of the surrounding gas which changes the Joule heating of the strip. To realize such sensors, not only materials with high temperature dependent electrical conductivity are required, but also minimization of the suspended strip dimensions is essential to maximize the responsivity and minimize the power consumption. Due to this, nanomaterials are especially attractive for this application. Here, we demonstrate the use of a multi-layer suspended graphene strip as a Pirani pressure sensor and compare its behavior with existing models. A clear pressure dependence of the strip’s electrical resistance is observed, with a maximum relative change of 2.75% between 1 and 1000 mbar and a power consumption of 8.5 mW. The use of graphene enables miniaturization of the device footprint by 100 times compared to state-of-the-art. Moreover, miniaturization allows for lower power consumption and/or higher responsivity and the sensor’s nanogap enables operation near atmospheric pressure that can be used in applications such as barometers for altitude measurement. Furthermore, we demonstrate that the sensor response depends on the type of gas molecules, which opens up the way to selective gas sensing applications. Finally, the graphene synthesis technology is compatible with wafer-scale fabrication, potentially enabling future chip-level integration with readout electronics.

Design and implementation of monolithically integrated sealed and unsealed chambers by using the wafer level packaging

This study presents the approaches to simultaneously realize various MEMS devices respectively encapsulated inside sealed and unsealed chambers by using the existing microfabrication and wafer level capping processes. In general, the wafer level capping process could encapsulate MEMS devices inside sealed chambers. In this study, the air leakage paths from the encapsulated chamber to the ambient have been selectively patterned and fabricated using the existing processes, so that the unsealed chambers are also achieved. The unsealed chamber is connected to the ambient and can be employed to encapsulate environmental monitoring devices such as humidity or pressure sensors. However, the sealed chamber is isolated with the ambient. The pressure in the sealed chamber is determined by the vacuum condition during the wafer level capping process. The sealed chamber could encapsulate devices such as motion sensors and resonators in the required vacuum condition. Thus, the presented approaches could enable existing process platforms to monolithically fabricate and encapsulate more devices for various applications. Devices such as micromachined Pirani gauge, resonator, and pressure sensor are fabricated and characterized to verify the present approaches. Measurement results indicate that sensors in different pressure conditions are respectively achieved by unsealed chambers and sealed chambers.

Elaboration of a Novel Design Pirani Pressure Sensor for High Dynamic Range Operation and Fast Response Time

We report a novel design for realizing Pirani sensor with a working range from a 1kPa up to pressure over than atmospheric one. The sensor is specifically designed to achieve high sensitivity, fast response time and high robustness. The proof of concept is composed of four metallic resistors interconnected to form a Wheatstone bridge. Two of them act simultaneously as the heating and sensing elements and the two others are used as a temperature reference. The heating element consists of a metallic wire of platinum Pt (3μm width, 1mm length) maintained on each lateral side by periodic silicon oxide SiO2 micro-bridges. The sensor design, fabrication technologies, electrical characterizations and voltage-pressure responses are described and shown. A future perspective is given, which describe the extension of this concept to elastic wave transduction of pressure using a combination of heater element and thin plate elastic waveguide.

Integration of a MEMS-type vacuum pump with a MEMS-type Pirani pressure gauge

The paper concerns the problems associated with the generation and measurement of vacuum in small-volume microelectromechanical (MEMS) devices. A concept of equipping vacuum MEMS with miniature vacuum pump and gauge has been presented. The ion-sorption vacuum micropump developed by the authors has been integrated with two “external” miniature pressure sensors: a specially developed miniature Bayard-Alpert gauge and a MEMS-type Pirani sensor. The pressure during the pumping process was in-situ measured and co-work of vacuum micropump and sensors has been investigated. The experimental results confirmed good pumping properties of the micropump, but indicated that the work of micropump significantly affects the operation of vacuum sensors and vice versa.

Extension of Operating Range Towards Lower Pressures of MEMS-based Thermal Vacuum Gauges by Laser-Induced Heating

Bulk micromachined Pirani pressure gauges have been processed on 1 μm thick silicon nitride membranes and on 1 μm thick nitride bridges resembling an H-shaped structure with the meander-shaped heating element positioned in the crossbar region of the H. For the membrane device, the resistor element is fabricated from 20 nm Cr/200 nm Au and for the H-shaped device from 30 nm Ni. The choice of using an H-shaped device and thinner, different metal was governed by the desire to increase the thermal resistances of the silicon nitride support structure and of the metal lines connecting the resistor element to the silicon cold junction, thus obtaining larger sensitivity and increased pressure range. It will be demonstrated that by heating the resistor elements via photon absorption using a green solid state laser as compared to Joules I2 R heating, the relative resistance change versus pressure dependency shifts by about one order of magnitude towards lower pressures for both device structures. This fact is desirable for measuring pressures in the 1x10-3 to 1x10-6 Torr range using these miniaturized gauges.

Wide vacuum pressure range monitoring by pirani SAW sensor

A new kind of surface acoustic wave (SAW) sensor has been developed to measure sub-atmospheric pressure below 100 mtorr with accuracy better than 0.1 mtorr. It provides an efficient measuring solution in the pressure range inaccessible in past by conventional diaphragm-based SAW sensors. Indeed, because of the small bending force in lower pressure and limited sensitivity, diaphragm-based SAW sensors are only suited to monitor relatively high pressure with a precision hardly better than 0.5 torr. To reach precision level better than 1 mtorr at sub-atmospheric pressure for vacuum technology applications, a radically different SAW-based solution is necessary. Our device aims to measure sub-atmospheric pressure less than 100 mtorr with a threshold resolution better than 0.1 mtorr. The concept is similar to the one used by Pirani pressure gauges. However, it is claimed that a heated and suspended SAW device should have better sensitivity. A theoretical model based on the basic concepts of gas kinetic theory and thermodynamics is presented. The validity of the model is checked by comparison between theoretical and experimental results.

Low-Temperature Monolithic Encapsulation Using Porous-Alumina Shell Anodized on Chip

A thin-film encapsulation process, featuring low-temperature steps, hermetic sealing (preliminary), and RF-compatible shell, is reported. Uniquely attractive as compared with the existing MEMS packaging approaches is its capability to monolithically package metal microstructures inside a microcavity on chip in one continuous surface-micromachining process. The key for this process is a technique to fabricate a large freestanding porous membrane on chip by postdeposition anodization of thin-film aluminum at room temperature. The porous-alumina membrane allows for the diffusion of gas or liquid etchants through the nanopores to etch away the sacrificial material underneath, freeing the movable microstructures encapsulated inside the cavity. To seal the package, a thin film is deposited over the alumina shell whose nanoscale pores of a high aspect ratio (> 30) do not allow any detectable penetration of the sealing material. The low-temperature (<300degC) encapsulation process produced a low-pressure seal (8 torr), monitored by a Pirani pressure gauge that also represents an encapsulated freestanding metal microstructure in the cavity. The thin-film package demonstrated a considerably low RF insertion loss of less than 0.1 dB up to 40 GHz.

The nanogap Pirani—a pressure sensor with superior linearity in an atmospheric pressure range

We have designed and fabricated a surface micromachined Pirani pressure sensor with an extremely narrow gap between its heater and heatsink (substrate) with superior output linearity in the atmospheric pressure range. The gap size of the device has been reduced to 50 nm by using a layer of PECVD amorphous silicon as a sacrificial layer and a xenon difluoride (XeF2) gas phase etching technique. Such a narrow gap pushes the transition from molecular to continuum heat conduction to pressures beyond 200 kPa. The higher transition pressure increases the measurement range and sensitivity of the gauge in atmospheric pressures. The gas phase etching of the sacrificial layer eliminates stiction problems related to a wet etching process. The active area of the sensor is only a 6 × 50 µm2 microbridge anchored to the substrate at both ends. An innovative fabrication technique was developed which resulted in a virtually flat microbridge with improved mechanical robustness. This process enabled us to have a very well-controlled gap between the microbridge and the substrate. The device was tested in a constant heater temperature mode with pressure ranges from 0.1 to 720 kPa. The heater power was only 3 mW at 101 kPa (atmospheric pressure), which increased to about 8 mW at 720 kPa. The output sensitivity and nonlinearity of the device were 0.55% per kPa at 101 kPa and ±13% of the output full scale, respectively.

Pirani pressure sensor for smart wafer-level packaging

Systems in Package (SiP) contain an increasing number of MEMS components such as resonators for time references or RF filtering. These resonators are packaged at wafer level and the cavity quality has to be guaranteed in terms of pressure and moisture level by the final manufacturing test. Therefore, environmental sensors could be integrated in the cavity to monitor these parameters and/or to calibrate the MEMS resonator with an electronic loopback. This paper presents the design of a low-cost Pirani pressure sensor for such application. First, the sensor principle is explained together with the fabrication process. Then, the closed-loop conditioning circuit, used to keep the sensor temperature constant, is described. It has been implemented on a first prototype that demonstrates a pressure measurement range extending over three decades, from 0.2 to 200 mbar. This range matches the cavity pressure level reached by some vacuum packaging techniques. In the end, the experimental results are used to develop a behavioral Matlab/Simulink model of the closed-loop sensor. This model provides simulation results in very good agreement with the experimental ones and it could be used to optimize the sensor design to cover an extended measurement range.

A microfabricated Pirani pressure sensor operating near atmospheric pressure

A micromachined pressure sensor based on the measurement of the pressure-dependent thermal conductivity of gaseous media (Pirani principle) is presented. The sensor consists of a freestanding microbridge with a defined distance to the substrate which acts as a heat sink. The theory to compute the necessary geometrical dimensions of the sensor element is presented. Due to a novel fabrication process, based on a sacrificial layer of ZnO, the sensor can readily be adapted to operate in different pressure ranges, even exceeding atmospheric pressure. A fully integrated bridge arrangement was used to compensate for ambient temperature changes. Experimental results for different sensor geometries are presented which correspond well with the theory. Basic design rules for micromachined Pirani pressure sensors are derived from the measurements.

Vapour pressure thermometry in high magnetic fields using a Pirani gauge

Vapour pressure thermometry has the advantage of not being affected by magnetic fields. Therefore, a study was carried out of thermometry below 1 K using a Pirani pressure gauge in the vapour of He 3. Its sensitivity and error in temperature measurements above 0.55 K are dV/dT ≥ K −1 and 0.4% respectively. Additionally, a semiconductor pressure sensor in the vapour of He 4 was examined for the vapour pressure thermometry.

A Study of Pfund’s Pressure Gage

Pfund’s combination of the McLeod and Pirani pressure gages was studied with a number of gases. The calibration curves of pressure against galvanometer reading were found to be straight lines and, except for hydrogen, nearly interchangeable for all the gases used. By using a galvanometer of high sensitivity and introducing a compensating filament, the sensitivity of the gage was increased until a change in pressure of 1.5×10−9 mm of mercury would cause a change in deflection of 1 cm on the galvanometer scale.Brief comments are made on the release of carbon monoxide and air from coconut charcoal and on the adsorption of air at low pressure by silica gel.

A method for the micro-analysis of gases by the use of the pirani pressure gauge

A method for the micro-analysis of gases by the use of the pirani pressure gauge

A Method for the Micro-analysis of Gases by the Use of the Pirani Pressure Gauge

A Method for the Micro-analysis of Gases by the Use of the Pirani Pressure Gauge